A main rotor of a helicopter rotates by driving power transmitted from an engine via a driving power transmission device. The main rotor includes a plurality of blades secured to a hub provided at a first end (one end) of a main shaft, and a second end (the other end) of the main shaft is supported by a main gear box included in the driving power transmission device in such a manner that the main shaft is rotatable. The driving power transmission device is provided with a mast bearing for supporting the main shaft in such a manner that the main shaft is rotatable. In a known configuration, the driving power transmission device includes a mast spacer disposed between the mast bearing and the mast nut to fix the mast bearing in an axial direction.
The mast spacer has a cylindrical shape covering the outer periphery of the main shaft. In a case where it is assumed that a portion of the main shaft which is closer to the hub (on the first end side) is an upper side and a portion of the main shaft which is closer to the main gear box (on the second end side) is a lower side, the upper portion of the mast spacer is in contact with the lower portion of the mast bearing. The lower portion of the mast spacer is in contact with a shaft sleeve. The lower portion of the shaft sleeve is fastened by use of the mast nut. By fastening the mast nut in an upward direction (from the lower side toward the upper side), the position of the mast bearing from the lower side toward the upper side is defined by the mast spacer. Thus, the position of the mast bearing in the axial direction of the main shaft is fixed by the mast spacer.
An exemplary mast spacer serves to fix the axial position of the mast bearing and suppress a creep of the mast bearing. In general, the creep is a phenomenon in which a bearing inner race slides (slips) and rotates relative to the main shaft, due to a radial load exerted on a bearing.
To avoid this, for example, projections protruding from the upper surface of the mast bearing are provided at the upper end of the both ends of the mast spacer, and recesses to which the projections are fittable are provided in the lower surface of a bearing inner race. In addition, similarly, projections are provided at the lower end of the mast spacer and recesses to which the projections are fittable are provided in the shaft sleeve. In this configuration, the upper portion of the mast spacer and the lower portion of the mast bearing mesh with each other, and the lower portion of the mast spacer and the shaft sleeve mesh with each other. As a result, occurrence of the creep can be suppressed.
If a great bending deformation of the main shaft occurs, a gap (clearance) is formed between the main shaft and the mast spacer, and a relative motion (movement) between the main shaft and the mast spacer occurs. This results in an abrasion (wear) occurring in the end surface or the projections of the mast spacer, as disclosed in Non-patent Literatures 1 and 2. To avoid occurrence of the abrasion, Patent Literature 1 discloses a bellows-like buffering structure (bellows section) provided at the upper portion of the mast spacer. By providing such a buffering structure, it becomes possible to prevent formation of a gap between the main shaft and mast spacer. In this way, occurrence of the abrasion (wear) in the end surface or the projections of the mast spacer can be effectively suppressed.